Similar magnetic fields despite different sizes

At the center of our Milky Way lies a huge black hole: Sagittarius A*, which contains the mass of around 4 million suns. A team of astronomers from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the Goethe University Frankfurt am Main, the Center for Astrophysics at Harvard and other international institutes has now examined this object in polarized radio light.

Using this method, scientists have now been able to detect a strong and organized magnetic field spiraling out from the edge of Sagittarius A* (Sgr A*).

In 2022, the first photo of Sagittarius A* was published by scientists from the global radio telescope network EHT (Event Horizon Telescope). The publication of the first photo of a black hole – namely that of M87* at the center of the galaxy Messier 87 – was presented to the public in 2019 under the direction of astrophysicist Heino Falcke.

M87* is over 53 million light-years away from us and is significantly more massive and a thousand times larger than Sgr A*. Yet both black holes look remarkably similar, the current research group notes.

So do the two supermassive black holes share other characteristics beyond their appearance?

Previous studies have already shown that the massive black hole M87* is able to eject powerful jets of material into the surrounding area – thanks to its magnetic field. Sagittarius A* is located about 27,000 light-years from us and is far smaller and lower in mass than M87*. The new images also indicate a strong magnetic field.

“What we see now is that there are strong, twisted and organized magnetic fields near the black hole at the center of the Milky Way Galaxy,” explains project co-leader Sara Issaoun (Harvard). By comparing it to M87*, the team “learned that strong and ordered magnetic fields are crucial to how black holes interact with the gas and matter around them.”

Light propagates as an oscillating or moving electromagnetic wave. We can only perceive objects when the light reaches us. Sometimes the light also oscillates in a preferred and therefore polarized orientation – which the human eye cannot distinguish from normal light. These particles swirl around the magnetic field lines that are in the plasma around the black holes. In doing so, they create a polarization pattern that is perpendicular to the field.

The polarization information of the light tells the astronomical community a lot about the physical properties of the gas and the mechanisms involved in feeding a black hole – and not just how strong the overall intensity is. This means that the processes in the regions of the black holes can be observed ever more closely. The magnetic field lines can also be mapped.

“Visualizing black holes in polarized light is not as easy as putting on polarized sunglasses,” explains Maciek Wielgus from MPIfR. The gas and plasma surrounding the black hole orbit Sgr A* within a few minutes. As the particles of the plasma swirl around the magnetic field lines, the magnetic field structures change rapidly as the EHT records the radio waves. Imaging is complicated by these rapid changes.

“Our polarized image of Sgr A* was the result of a careful comparison between the actual measurements and the hundreds of thousands of possible image variants that we can create using advanced supercomputer simulations,” says theoretical astrophysicist Luciano Rezzolla of Goethe University Frankfurt.

The original image of Sgr A* was also a composite of several snapshots, explain Rezzolla and Wielgus. The new polarized images therefore also represent a kind of average of all measurements.

EHT deputy project scientist Mariafelicia De Laurentis (University of Naples Federico II, Italy) concludes that the differences in mass, size and environment among supermassive black holes may be universal. Nevertheless, the physical processes that control a black hole’s feeding and jet ejection appear to be identical.